1. Field of the Invention
This invention relates to analog-to-digital converters and, more particularly, to an analog-to-digital converter system for correcting the overload of an analog to digital (A/D) converter.
2. Description of Related Art
Many physical devices generate output signals which are analog or continuously varying. Today, signal processing is often accomplished using digital methods. In many applications it is required to convert an analog signal into a digital form suitable for processing by a digital system. Many types of converters exist which act as interfaces between analog devices and digital systems. These converters are used in a variety of applications, including testing, measurement, process control, and communications. Analog-to-Digital (A/D) converters produce a digital output from an analog input. In converting analog signals to digital form, the analog signal is typically sampled and quantized. When the analog input signal to the A/D converter reaches above the full scale voltage level producing the maximum digital output value for the A/D converter, the A/D converter becomes saturated or overloaded. Once the A/D converter is saturated, the digital output cannot go above the maximum digital output value which is limited by the number of bits available at the output of the A/D converter. As the analog input signal increases above the full scale voltage level, the sudden clipping in the digital output pattern results in a massive spurious response or undesirable distortion, which can be referred to as a discontinuity, in the digital domain when a Fourier transform is taken of the digital output signal resulting from the analog input signal with the sudden clipping of the amplitude.
The present invention involves an analog-to-digital (A/D) converter system which converts an attenuated analog input signal into a digital value and shifts the position of the digital value within the digital output depending on the attenuation of the analog input signal. For example, in response to an analog input signal which saturates a first A/D converter, a second A/D converter receives and converts an attenuated analog input signal, and the output of the second A/D converter is used to produce the more significant bits of the digital output value to convert a higher amplitude range (power or voltage level range) for the analog input signal. In another example, a level detector can detect the amplitude of the analog input signal and provide an indication of the amplitude of the analog input signal. In response, the analog input signal is attenuated and provided to an A/D converter. The A/D converter converts the attenuated analog input signal and produces a digital value as the bits of the digital output where the position of the digital value within the digital output depends on the attenuation of the analog input signal.